There have been conventionally known wired LANs as a form of networks, whereas in recent years wireless LANs have been in common use instead of wired LANs. The wireless LANs are used, for example, at Hotspots on streets, at offices, and homes.
At a Hotspot, users can access the Internet freely just by connecting a wireless LAN adapter to a notebook-sized personal computer or using PDA (Personal Digital Asistant) without becoming a member of a certain service or installing a dedicated software.
Also, at offices and homes, users can connect notebook-sized personal computers, hereinafter referred to as PCs, to broadband routers by wireless everywhere to access the Internet freely just by connecting wireless LAN adapters to their PCs.
The notebook-sized PCs and PDAs each incorporate an antenna for sending and receiving radio waves. Further, as wireless LAN standards, there are IEEE 802.11b according to IEEE (Institute of Electrical and Electronic Engineers) as well as Bluetooth for short-range radio communication, for example.
A notebook-sized PC may include a plurality of antennas in which a diversity system is adopted for the purpose of increasing a receiving efficiency (see JP-A-2003-37538, for example). The notebook-sized PC has two antennas for receiving through the diversity system and an antenna for receiving through a radio communication system different from the former antennas, which are effectively disposed in a limited space inside the case. Moreover, these internal antennas are provided at as high positions as possible inside a device case, i.e., at upper portions of the device case in order to improve receiving sensitivities of the antennas.
However, because the notebook-sized PC disclosed by JP-A-2003-37538 has an antenna provided within a device case, the case may block radio waves electromagnetically. In this case, it is necessary to shift the location of the device in order to increase the sensitivities to radio waves.
To solve the problem, there has been proposed a structure such that a coaxial connector with a switching function is provided on the device case to connect an external antenna to the coaxial connector. According to this proposal, connecting the internal and external antennas selectively to a radio unit enables the sending and receiving of radio waves through the antenna with a better sensitivity.
Here, it is common that the external antenna and the coaxial connector, to which the external antenna is connected, are designed so as to have an impedance of 50 ohms for the purpose of the impedance matching between them.
However, their impedances cannot be always set to 50 ohms easily because the coaxial connector is restricted in shape. Further, setting the impedances to 50 ohms cannot necessarily reduce the transmission loss according to the frequency characteristics, which can make it difficult to pick out a required frequency effectively.
In the meanwhile, the radio communication system which is applied to wireless LANs, etc. is changing rapidly. For example, a 2.4 GHz frequency band is standardized under IEEE 802.11; a frequency band around 5.2 GHz is standardized under IEEE 802.11a. In addition, a 2.4 GHz frequency band is standardized under IEEE 802.11b; and a 2.4 GHz frequency band is standardized under IEEE 802.11g. Also, it is desired to standardize a frequency band around 5.8 GHz in the future.
Of these frequency bands, in case of a high frequency band of the order of 5 GHz, the internal antenna and radio unit may not be matched in impedance even when the external antenna and radio unit can be matched in impedance. Therefore, there is needed an impedance matching circuit for making the internal antenna and radio unit match in impedance.
However, the impedance matching circuit is a high frequency circuit, which is provided separately from a radio unit to be mounted inside the device case and a control unit for processing data. On this account, wireless communication device manufacturers have to design the impedance matching circuit whenever the applied frequency band is changed, which is inconvenient.
In order to solve the problems, it is an object of the present invention to provide a connector unit, which is used for an apparatus for switching internal and external antennas and is capable of matching the external and internal antennas in impedance.
To achieve the object, a novel connector unit as below is invented.